Casting polymerization is known as a method of molding a plastic lens. Casting Polymerization is a method of molding a plastic lens by filling a lens raw material liquid (to be referred to as a monomer hereinafter) into the cavity of a mold for molding the plastic lens and heating and polymerizing the monomer at a predetermined temperature to cure it (e.g., “Spectacles” Published by Medical Aoi Shuppan, May 22, 1986, pp. 83 to 85). Usually, a mold for molding a plastic lens comprises a pair of optical surface forming molds which form the optical surfaces (a convex surface and concave surface) of the plastic lens and a cylindrical gasket in which the molds are to be fitted at a predetermined gap from each other (e.g., Japanese Patent Publication No. 58-45940, Japanese Utility Model Publication No. 5-18107, Japanese Patent Publication No. 6-98631, and Japanese Utility Model Publication No. 6-39951).
As shown in FIGS. 26 to 28, a mold 200 for molding a plastic lens described in Japanese Patent Publication No. 58-45940 comprises first and second circular molds 201 and 202 and a cylindrical gasket 203 in which the molds 201 and 202 are to be incorporated.
The first mold 201 located on the upper side has a first lens molding surface (convex surface) 204a to mold a lens back surface (concave surface). The second mold 202 located on the lower side has a second lens molding surface (concave surface) 205a to mold a lens front surface (convex surface). The molds 201 and 202 are incorporated in the gasket 203 such that the lens molding surfaces 204a and 205a oppose each other.
The gasket 203 comprises a cylindrical gasket main body 203A, an annular overhang portion 203B which integrally projects on the central portion of the inner circumferential surface of the gasket main body 203A throughout the entire circumference, a collar portion 203C which integrally projects on the upper shoulder of the overhang portion 203B throughout the entire circumference, and a corner portion 203D which integrally projects on the lower shoulder of the overhang portion 203B throughout the entire circumference. A plurality of notches 206 are formed in each of the upper and lower end openings of the gasket main body 203A at necessary intervals. The collar portion 203C has an acute triangular sectional shape and integrally projects obliquely upward on the inner side surface at the upper end of the overhang portion 203B. The collar portion 203B is elastically deformable in the direction of plate thickness. The corner portion 203D does not project inside the overhang portion 203B but projects only downward.
To assemble the mold 200, first, the second mold 202 is placed on a support plate 207 such that its second lens molding surface 205a faces upward. The gasket 203 is inserted in the second mold 202 such that the lower opening of the gasket 203 matches the outer portion of the second mold 202, and the second mold 202 is forced into the gasket 203 until the distal end of the corner portion 203D comes into contact with the second lens molding surface 205a. A monomer is filled in a recess formed by the overhang portion 203B of the gasket 203 and the second lens molding surface 205a of the second mold 202. After that, the first mold 201 is fitted in the upper opening of the gasket 203, and the first lens molding surface 204a is urged against the collar portion 203C to elastically deform the collar portion 203C downward. When the first mold 201 is forced into the gasket 203, the excessive monomer in the gasket 203 overflows outside the gasket 203 from the notches 206 through the gaps between the gasket 203 and the first and second molds 201 and 202. At this time, when the force to push the first mold 201 into the gasket 203 is released, the downwardly flexed collar portion 203C is to be elastically restored upward. Thus, the inner space (cavity) of the gasket 203 which is sealed off by the first and second molds 201 and 202 enlarges, and its pressure becomes negative so as to hold the first and second molds 201 and 202 in tight contact with the collar portion 203C and corner portion 203D. Consequently, the interior of the gasket 203 is sealed well.
FIGS. 29A and 29B show a mold 300 for molding a plastic lens described in Japanese Utility Model Publication No. 5-18107. The molding mold 300 comprises first and second circular molds 301 and 302 and a gasket 303 into which the molds are to be pressed. The first mold 301 has a first lens molding surface (convex surface) 304a to mold a lens back surface (concave surface). The second mold 302 has a second lens molding surface (concave surface) 305a to mold a lens front surface (convex surface).
The gasket 303 comprises a cylindrical gasket main body 303A and a ring-like projecting band 303B which integrally projects on the inner circumferential surface of the gasket main body 303A. The ring-like projecting band 303B has a vertical inner circumferential surface, and its upper end portion 306 has an acute-angled hill-like sectional shape. The inner diameter of the gasket main body 303A is equal to or slightly smaller than the outer diameter of each of the first and second molds 301 and 302 which are to be fitted and inserted in the gasket main body 303A. To incorporate the first mold 301 in the gasket 303, the first mold 301 is pressed into the gasket 303 from the upper opening and forced until the first lens molding surface 304a comes into contact with the distal end of the upper end portion 306 of the projecting band 303B. At this time, the gasket main body 303A of the gasket 303 is stretched out in the diameter-increasing direction. When the force that presses the first mold 301 downward is released, the circumferential surface of the first mold 301 is kept clamped by the inner circumferential surface of the gasket main body 303A due to the shape restoring force of the gasket main body 303A, and the first mold 301 is held by the gasket main body 303A with the first lens molding surface 304a being in contact with the distal end of the upper end portion 306 of the projecting band 303B.
FIGS. 30 to 34 show still another conventional example of the mold for molding a plastic lens. In this conventional example, the inner circumferential surface of a gasket seals the circumferential surface of a mold. A mold 400 for molding a plastic lens comprises a cylindrical gasket 401 and first and second molds 402 and 403 which are to be fitted in the gasket 401.
The gasket 401 comprises a cylindrical gasket main body 401A which is made of an elastic plastic material, a ring-like projecting ridge 401B which integrally projects on the inner circumferential surface of the gasket main body 401A, and a monomer-filling filling port portion 401C which integrally projects on the outer circumferential surface of the gasket main body 401A.
The first and second molds 402 and 403 are cylindrically formed of glass. The first mold 402 to be incorporated on the upper side has a first lens molding surface 404a to mold the front surface (convex surface) of the lens. The second mold 403 to be incorporated on the lower side has a second lens molding surface 405a to mold the back surface (concave surface) of the lens. When the first and second molds 402 and 403 are pressed into the gasket 401 and incorporated in it such that the lens molding surfaces 404a and 405a oppose each other, circumferential surfaces (edge surfaces) 417 and 418 come into tight contact with an inner circumferential surface 406a of the gasket main body 401A to seal the gaps between the gasket main body 401A and the first and second molds 402 and 403, so as to prevent a monomer 410 to be filled into the gasket main body 401A from leaking and the atmosphere from entering. The upper and lower surfaces of the projecting ridge 401B serve to regulate the insertion amounts of the first and second molds 402 and 403 and position them.